2.4.1

Water Structure & Function

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Function of Water

Water is a major component of cells and makes up 60-70% of the human body. Life evolved in an environment where water was abundant. It has several properties that are important in biology.

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Importance of water

  • Water is one of the most useful molecules for life. Its uses include:
    • As a reactant in cells (e.g. photosynthesis, hydrolysis).
    • Provides structural support in cells.
    • Keeps organisms cool to maintain an optimum body temperature.
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Properties of water

  • Special properties of water are:
    • Metabolic importance.
    • High heat capacity.
    • Heat of vaporization.
    • Cohesive properties.
    • Useful as a solvent.

The Structure of Water

The structure of a water molecule helps us to understand hydrogen bonding and the function of water.

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Contents of a water molecule

  • Water molecules (H2O) are made from:
    • One oxygen atom.
    • Two hydrogen atoms.
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Polarity

  • Water is a polar molecule.
    • The oxygen atoms in water are slightly negatively charged.
    • The hydrogen atoms in water are slightly positively charged.
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Hydrogen bonding

  • The polarity of water molecules means that a hydrogen atom on one water molecule is attracted to the oxygen atom on another water molecule.
  • This attraction is called hydrogen bonding.

Useful Properties of Water

Water is a major component of cells and is essential to life as we know it (60–70% of the human body is made up of water). The properties of water that make it such a useful substance are:

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High latent heat of vaporisation

  • The latent heat of vaporisation is the amount of energy needed to change one gram of a liquid substance to a gas.
  • As liquid water heats up, hydrogen bonding makes it difficult to separate the water molecules from each other. This means that a lot of energy is needed for water to evaporate.
    • When water evaporates, energy is used up - this cools the environment where the evaporation is taking place.
    • This is why sweating helps with body temperature regulation.
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High heat capacity

  • Specific heat capacity is the amount of heat one kilogram of a substance must absorb or lose to change its temperature by 1oC.
  • Water has a high heat capacity so takes a long time to heat and cool.
    • The specific heat capacity of water is much larger than sand. This is why land cools faster than the sea.
    • Water is used by warm blooded animals to more evenly disperse heat in their bodies.
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Good solvent

  • Water is a good solvent because ions and polar molecules can easily dissolve in it.
  • Water is a polar molecule. This means that the positive end of the water molecule attracts negative ions and the negative end will attract positive ions.
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Good metabolite

  • Water is used or formed in many metabolic reactions, such as condensation and hydrolysis reactions.
    • ATP + H2O → ADP + Pi + energy
      • A bond is broken and a water molecule is used up and so this is a hydrolysis reaction.
    • ADP + Pi + energy → ATP + H2O
      • A new bond is formed and a water molecule is released and so this is a condensation reaction.
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Cohesive properties

  • The strong attraction between water molecules due to hydrogen bonds is called cohesion.
  • Cohesion produces surface tension where water meets air.
    • This is why water forms droplets when placed on a dry surface rather than being flattened out by gravity.
    • Plants use this natural phenomenon to help transport water from their roots to their leaves.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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